A conventional hydraulic clutch control for a motor vehicle friction clutch has a master cylinder connected with a compensating container filled with hydraulic fluid, which master cylinder can be actuated via a clutch pedal or an electric motor drive. The master cylinder is hydraulically connected with a slave cylinder via compressed-air piping, so that by depressing the clutch pedal or causing displacement of the master cylinder by electric motor, pressure generated in the master cylinder can be transmitted via the head of fluid in the compressed-air piping to the slave cylinder. As a result, the release bearing of the friction clutch is subjected to an actuating force via the slave cylinder piston in order to separate the clutch pressure plate from a clutch carrier plate via a releasing device and so to separate the engine from the motor vehicle gears. Hydraulic clutch controls are also known, the releasing device of the friction clutch being capable of being actuated with an operating or slave cylinder operatively connected to them, which in turn is controlled via a hydraulic servo circuit.
In order to guarantee the most evenly possible actuation of the friction clutch with low space requirement for the operating or slave cylinder, the state of the art has also suggested that the slave cylinder be formed as an annular cylinder arranged around the clutch or gear shaft and secured to the gears. In the annular cylinder an annular piston is arranged to slide in an axial direction of the clutch or gear shaft which is operatively connected with the release bearing of the friction clutch. When the annular cylinder impacts hydraulically via the compressed-air piping the annular piston acts via the release bearing on the releasing device of the friction clutch in order to disengage this. Such slave cylinders are also known as central disengaging or releasing devices due to their arrangement concentric to the clutch or gear shaft.
With such hydraulic clutch controls, providing sensor devices on the clutch actuation side is already known, in other words without intervention at the actual components of the friction clutch being necessary, in order to sense indirectly the position of the clutch pressure plate. Such sensing permits on the one hand a statement to be made as to the state of wear on the friction clutch. On the other hand, the disengagement respectively engagement state of the friction clutch can be determined, so that, for example, in the case of automated friction clutches, the clutch actuation can be controlled automatically to the desired extent which, for example, permits a reduction in clutch switching times. Finally, the results of sensing can also be used for safety measures during operation of the motor vehicle, for example, so that the motor vehicle can only be started once the clutch pedal is depressed.